Append the string body to the end of the string currently
being concatenated as the last string in the string buffer,
and update its count field. If there is no concatenating
string, start one. An error is thrown if there is not
enough room in string space even after a garbage collection.
When there is a concatenating string, CAT and
the only basic string operations that can copy a string into
the string buffer. Pushes onto the string stack without copy
are still allowed.
NOTE: It is left to the user to define special concatenating
: \n-cat ( -- ) \n$ cat ;
dstrings ordinary primitive